Process of protecting castings made of carbon-containing alloys against decarburization at the edges and against surface defects



United States Patent Ofifice Patented on. 7, 1969 US. Cl. 164-138 3 Claims ABSTRACT OF THE DISCLOSURE Castings made of carbon-containing alloys are protected against decarburization due to reaction with oxygen present in the molds by incorporating in the mold inhibitors such as alloys of aluminum and silicon or silicides and mixtures thereof which have a melting point below that at which the casting is poured but at least above the firing temperature of the mold (above about 800 C.).

The present invention pertains to a method of protecting castings made of carbon-containing alloys against decarburization at the edges thereof and against the formation of surface defects during cooling in a mold which is fired prior to casting.

Molds of this type are much used in the manufacture of precision castings. These molds are formed about a model or pattern of the piece to be cast, which pattern is melted, dissolved or otherwise destructively removed from the mold formed about it. When such molds are used in the casting of carbon-containing alloys, especially steels having high carbon content, the edge regions of the castings may under certain circumstances decarburize to an undesirable degree during cooling. At the same time surface defects often appear, especially in the form of pittings. Such phenomena are particularly noticeable when the melt is poured into the mold while the mold is hot, as is customary in precision casting operations, the molds being in such cases removed from the furnaces in which they are fired immediately prior to the casting operation itself. The decarburization and the appearance of the surface defects above referred to are consequences of the fact that air diffuses through the mold and comes into contact with the edge regions of the casting. As a result of this diffusion, part of the carbon in the edge portions of the casting may be oxidized. Oxidation products of iron and its accompanying elements may lead to the formation of pittings particularly in the case of 13 percent chromium steel. The term 13 percent chromium steel is used to refer to that class of steels comprising,

in weight percents:

with the balance iron and usual incidental elements such as Si, Mn, S and P. These dangers are particularly present when thick-walled and complicated castings are to be produced, such castings remaining above 800 C. for relatively long times during the cooling process.

It has already been proposed to combat these undesirable developments by inclusion of an inhibiting or protective material in the pattern employed for production of the casting mold. The protective material is so chosen that it tends to capture or combine with oxygen durir g cooling of the casting at temperatures above those at tit) which the mold has been fired. Thus in particular in this prior art process carbides or cyanides such as calcium carbide and barium cyanide have been proposed.

The invention provides a great and striking improvement in the protective effect achieved. According to the invention there is employed a protective material whose melting point is below that at which the casting is poured but which lies above the firing temperature of the mold, which firing temperature is at least 800 C.

The increased protective effect of the material employed in accordance with the invention rests upon the fact that its susceptibility to reaction, after it has been liquefied by the heat of the melt, is much larger than that of known protective agents which are in the solid state in the mold when they develop their protective action. By means of the invention it is therefore possible to produce excellent castings, even of especially sensitive steels, without appearance of deleterious decarburization or surface pitting.

In accordance with the invention the protective material preferably comprises a silicide or a mixture of silicides. Particularly good results have been obtained with calcium silicide whose melting point is in the range from about 920 C. to 1220 C. Alloys of aluminum and silicon, especially those having a content of at least 30 percent aluminimum and 20 percent silicon, are other good protective materials for use in the invention.

The invention will nOW be further described with reference to a number of examples.

EXAMPLE 1 250 kilograms of finely ground fire clay, 25 kilograms of refractory cement, and 25 kilograms of CaSi are mixed with Water to form a slurry-like mold material. A wax pattern of the piece to be cast, coated in known fashion with a refractory overlay of granular zirconium and aqueous colloidal silicic acid is positioned in a flask and the flask is filled with the mold material around the pattern. The mold so obtained is fired overnight at a temperature of about some 900 C., in the course of which firing the wax pattern is destroyed. Directly upon removal of the mold from the firing furnace, liquid 13 percent chromium steel at a temperature of about 1600 C. is poured into the mold. The melting point of the calcium silicide in the mold is in the vicinity of 1220 C. so that it will have remained in the solid state during the firing of the mold, in which state it is relatively inert in the face of the oxygen present in the atmosphere of the firing furnace. It is consequently unnecessary to maintain a reducing or inert atmosphere in that furnace. The composition of the steel used in this example was, in weight percents:

with the balance iron.

Under the influence of the heat of the molten metal cast, the temperature of the mold is suddenly increased, with consequent fusion of the calcium silicide therein. When in the fused condition, the calcium silicide is extremely reactive and combines with practically all of the oxygen which diffuses through the pores of the mold and which in the absence of the protective calcium silicide material would produce a deleterious decarbun'zation and surface pitting in the casting. The finished casting is characterized by a uniform distribution of carbon therein, including the surface layers thereof, and it exhibits an excellent surface quality substantially free from surface pitting.

In place of CaSi it is possible to use other calcium silicides such as Ca Si having a fusion point of 920 C. or CaSiO having a fusion point of 1020 C., or mixtures of these silicides. Good results are obtained with from 2 to 15 and especially from 4 to 10 percent by weight of the protective material, taken by reference to the dry weight of the mixture from which the mold is made.

EXAMPLE 2 A mold mixture is prepared from a solution of the liters of ethyl silicate, 2.5 liters of alcohol, 0.25 liter of water, 5 cubic centimeters of 32 percent hydrochloric acid and 18 kilograms of granular zirconium.

A pattern of the piece to be cast made of urea is dipped into the mold mixture so produced and, after drying of the coating so obtained, this process is repeated. Immediately after the fourth dipping the surface is scattered over with a finely ground aluminum-silicon alloy to provide a protective material. This aluminum alloy comprises about 0.8 percent carbon, 33 percent silicon, 48 percent aluminum, 3 percent titanium, 13 percent iron and the usual accompanying elements and impurities and is commercially available under the name Alsimin. This powdering of the surface is repeated after the fifth, sixth, seventh and eighth dippings, the eighth dipping being the next to last. The quantitiy of protective material so incorporated amounts to about 14 percent by weight of the dry weight of the material in the finished shell-like mold thus obtained.

The shell-type mold thus obtained is allowed to dry and solidify for several hours in air and is then immersed in water which dissolves out the urea pattern at the core. This dissolving can be allowed to proceed to completion or only part way, in which latter case the remainder will be removed upon firing of the mold. Firing is effected in a furnace for 6 hours at a temperature of 900 C. The protective material existing between the dipped layers maintains its solid condition during the firing since its melting temperature is about 1100 C.

After removal from the furnace, the mold is poured with a carbon-containing steel having a melting temperature of 1600 C. Under the influence of the heat in the melt, the aluminum-silicon alloy fuses and is effective as a protective against edge decarburization. Preferable reducing elements are aluminum, silicon and titanium. The piece so cast is free of all edge decarburization and shows a very good surface quality.

The invention is not limited to the example described. Other methods may be employed for production of the mold such as those in which the pattern, without any dip-formed coatings thereon, is packed in a mold flask with a mold material having the protective substance uniformly distributed through it.

The invention thus provides a method of protecting castings of carbon-containing alloys against decarburization and the formation of surface defects during cooling in a previously fired mold. According to this process, there is included in the mold a material which is capable of combining with oxygen when, during cooling of the melt, that material is heated to a temperature above that at which the mold is fired, the material having a melting temperature above 800 C. and above the temperature at which the mold is fired, but below the temperature at which the melt is poured. In another aspect thereof, the inventin provides a process of casting carbon-containing alloys according to which there is made a mold including a material having a melting temperature above 800 C. and having an afiinity for oxygen when heated above its melting temperature. Further according to this process of casting, the mold is fired at a temperature of at least 800 C. but below the melting temperature of that material, and there is poured into the mold a molten carbon-containing alloy heated to a temperature above that melting temperature.

While the invention has been described hereinabove in terms of the presently preferred practice thereof, the invention itself is not limited thereto but comprehends all variations thereon and departures therefrom properly falling within the spirit and scope of the appended claims.

I claim:

1. A process of protecting castings of carbon-containing alloys against decarburization and the formation of surface defects during cooling in a mold comprising making a porous mold and including in the body thereof a solid protective reducing material having a relatively low afiinity for oxygen when said material is in its solid state, said material becoming liquefied at temperatures considerably above 800 C. and having when liquefied a relatively great afiinity for oxygen, said material comprising a mixture of silicides, CaSi, Ca Si and CaSiO and choosing a mixture thereof to vary the fusion point of said protective material, firing said mold before casting to a temperature of at least 800 C. and introducing a melt into said mold having a temperature considerably above 800 C., said protective material becoming liquefied in the temperature interval between the temperature at which the melt is poured and the temperature of at least 800 C. at which the mold is fired.

2. A process of protecting castings of carbon-containing alloys against a decarburization and the formation of surface defects during cooling in a mold comprising making a porous mold and including in the body thereof a solid protective reducing material having a relatively low afiinity for oxygen when said material is in its solid state, said material becoming liquefied at temperatures considerably above 800 C. and having when liquefied a relatively great affinity for oxygen, said material comprising an aluminum-silicon alloy wherein the aluminum content comprises at least 30%, firing said mold before casting to a temperature of at least 800 C. and introducing a melt into said mold having a temperature considerably above 800 C., said protective material becoming liquefied in the temperature interval between the temperature at which the melt is poured and the temperature of at least 800 C. at which the mold is fired.

3. A process according to claim 2 wherein said alloy includes by weight at least 30% aluminum and 20% silicon.

References Cited UNITED STATES PATENTS 2,948,034 8/ 1960 Schneider 164-41 3,215,525 11/1965 Sprankle 58 X 3,286,312 11/1966 Davis et al. 249-114 X FOREIGN PATENTS 1,117,263 11/1961 Germany.

I. SPENCER OVERHOLSER, Primary Examiner R. SPENCER ANNEAR, Assistant Examiner US. Cl. X.R. 

